86 ANIMAL BIOLOGY 



On either side of the thorax, in a space called the bran- 

 chial chamber which is covered over by the sides of the 

 carapace, are the breathing organs or gills (branchice). 

 The crayfish breathes the air which is dissolved in the water. 

 A fresh supply of water is kept passing over the gills by 

 the movements of a lobe on the second maxilla; the water 

 is expelled from the anterior part of the gill cavity and 

 passes into it from the sides under the edge of the carapace. 

 If one places a little colored fluid in a dish containing a 

 crayfish the fluid can be seen to be drawn into the chamber 



br 



FIG. 73. Anatomy of the crayfish, a 1 , first antenna; a 2 , second 

 antenna; br, brain; d.l., duct of liver; e, eye; g, green gland or excretory 

 organ opening at g l , h, heart; *, intestine, I; liver; m, mouth; ms, muscles; 

 P, pericardium or sac surrounding the heart; r, rostrum; s, stomach; sa, 

 sternal artery; v.n., ventral nerve cord. (After Hatschek and Cori.) 



and expelled in a stream in front of the body. The gills 

 of the crayfish are feather-like structures consisting of a 

 large number of filaments attached to a central stem or 

 axis. Some of the gills are situated on the bases of the 

 walking legs, others are attached to the sides of the body. 

 They are to be regarded as complex and greatly branched 

 out-pushings of the surface of the body in order to afford 

 a great increase of surface exposed to the water; the walls 

 of the filaments .are very thin in order to facilitate the 



